The present invention relates to a sensor using two tunable oscillators connected to a frequency mixer and comprising a device for calibrating the frequency of the output signal and the process for calibrating this frequency.
The invention relates in particular to surface acoustic wave pressure gauges.
It is known to use the surface acoustic waves propagated between two transducers with interfitting electrodes in a piezoelectric substrate to measure accelerations, stresses or pressures exerted on a thin membrane contained in this substrate. To this end, the delay line formed between two transducers is relooped by means of an amplifier and forms an oscillator whose frequency varies depending on the variations of stress in the materials.
It is also known to associate two oscillators of this type, by implanting on the same membrane two delay lines and in combining the output frequencies of these two oscillators so that a variation in the stresses applied to the membrane produces frequency variations in the opposite direction. The output frequency of the pressure gauge thus formed is equal to the beat between these two frequencies. This process compensates in a large part particularly for the effect of temperature drifts, the temperature drifts causing variations in frequency of the same sign in both oscillators. A device of this type is described for example in U.S. Pat. No. 4,100,811.
For some types of application, it is necessary for the output frequency of the pressure gauge to be other than zero in the rest condition, i.e. in the absence of any stress. This is particularly the case for a type of pressure gauge used in the field of automobile electronics.
This pressure gauge allows depression measurements to be effected with respect to the atmospheric pressure and may in particular be used for controlling an internal combustion engine. The frequency of the two oscillators forming the pressure gauge (and so the output frequency of the pressure gauge which is the difference between these two frequencies) depends on a number of parameters, in particular on the nature of the crystal used, on the nature of the connections and on the characteristics of the electric circuits used.
For so-called general public applications, economic considerations impose the use of cheap elements. It follows that, for example, amplifiers forming the associated electronic circuits present dispersions in particular in their characteristic parameters. The direct result is that the value of the peak frequency in the rest condition is not obtained in a reproducible manner from one sample to the next.